With 3GPP LTE, OFDMA (Orthogonal Frequency Division Multiple Access) is adopted as a downlink communication scheme. In a radio communication system supporting 3GPP LTE, a base station transmits synchronizing signals (synchronization channels: SCHs) and broadcast signals (broadcast channels: BCHs) using predetermined communication resources. Then, a terminal first captures an SCH to secure synchronization with the base station. Then, the terminal acquires parameters (e.g. a frequency bandwidth and so forth) unique to the base station by reading BCH information (see Non-Patent Literatures 1 and 2).
After completing acquisition of the parameters unique to the base station, the terminal requests the base station for a connection to establish communication with the base station. The base station transmits control information to the terminal having established communication with the base station using PDCCHs (physical downlink control channels) if necessary.
Then, the terminal performs “blind detection” on a received PDCCH signal. That is, a PDCCH signal contains a CRC part, and, in the base station, this CRC part is masked with the terminal ID of the terminal to which the PDCCH signal should be transmitted. Therefore, the terminal cannot determine whether or not the PDCCH signal is directed to the terminal until demasking the CRC part of the received PDCCH signal with the terminal's ID. With this blind detection, when CRC calculation is OK as the result of demasking, this PDCCH signal is determined as a signal directed to the terminal.
In addition, control information transmitted from a base station includes resource allocation information containing information about resources allocated from the base station to a terminal. A terminal needs to receive both downlink resource allocation information and uplink resource allocation information. These downlink resource allocation information and uplink resource allocation information are transmitted using PDCCH signals having the same size. A PDCCH signal contains the type information (e.g. one bit flag) about resource allocation information. Therefore, even if a PDCCH signal containing downlink resource allocation information and a PDCCH signal containing uplink resource allocation information have the same size, a terminal can distinguish between downlink resource allocation information and uplink resource allocation information by checking the type information about resource allocation information. Here, the format of PDCCHs used to transmit uplink resource allocation information is “PDCCH Format 0”, and the format of PDCCHs used to transmit downlink resource allocation information is “PDCCH Format 1A.”
However, a case is possible where the uplink bandwidth and the downlink bandwidth differ, and in this case, the size of information (that is, the number of bits required for transmission) is different between downlink resource allocation information and uplink resource allocation information. To be more specific, when the uplink bandwidth is small, the size of uplink resource allocation information is small, and, when the downlink bandwidth is small, the size of downlink resource allocation information is small. In this way, when the size of information varies due to a difference in bandwidth, zero information is added (that is, zero padding is performed) to either resource allocation information having a smaller size to make downlink resource allocation information and uplink resource allocation information have the same size. By this means, it is possible to keep the size of PDCCH signals the same regardless whether the information is downlink resource allocation information or uplink resource allocation information.
In addition, standardization of 3GPP LTE-Advanced that realizes communication faster than 3GPP LTE has been started. A 3GPP LTE-Advanced system (hereinafter, also referred to as “LTE-A system”) follows a 3GPP LTE system (hereinafter, also referred to as “LTE system”). With 3GPP LTE-Advanced, base stations and terminals that can perform communication in a wideband frequency equal to or higher than 20 MHz, will be introduced in order to realize a downlink transmission speed equal to or higher than maximum 1 Gbps.
Moreover, with 3GPP LTE-Advanced, a case is possible where the communication bandwidth of the uplink is not symmetrical to the communication bandwidth of the downlink, taking into account that there is a difference in the required throughput between the uplink and the downlink. To be more specific, with 3GPP LTE-Advanced, the communication bandwidth of the downlink may be wider than the communication bandwidth of the uplink.
Here, a base station supporting an LTE-A system (hereinafter, “LTE-A base station”) is configured to be able to perform communication using a plurality of “component bands.” “Component band” is a band having a width of the maximum 20 MHz, and is defined as the basic unit of communication bands. In addition, “component band” in the downlink (hereinafter referred to as “downlink component band”) may be defined as a band separated according to downlink frequency band information contained in a BCH broadcasted from a base station, or a band defined by the distribution width in which downlink control channels (PDCCHs) are distributed and assigned in the frequency domain. On the other hand, “component band” in the uplink (hereinafter “uplink component band”) may be defined as a band separated according to uplink frequency band information contained in a BCH broadcasted from a base station, or the basic unit of communication bands equal to or lower than 20 MHz including a PUSCH (physical uplink shared channel) around the center and including PDCCHs in both end parts. Here, in 3GPP LTE-Advanced, “component band (s)” may be written as “component carrier (s)” in English.
FIG. 1 shows an arrangement example of channels in an LTE-A system where the communication bandwidth (the number of component band s) for the uplink is not symmetrical to the communication bandwidth for the downlink. In FIG. 1, in order to make a terminal transmit an uplink signal, an LTE-A base station reports resource allocation information from both two downlink component bands, using PDCCHs. The uplink component band is associated with both downlink component bands, and therefore a PUSCH is transmitted using the same uplink component band even if uplink resource allocation information is transmitted using either downlink component band. In addition, downlink resource allocation information may be transmitted from both of two downlink component bands, and each downlink resource allocation information is used to indicate, to a terminal, a downlink-allocated resource in the downlink component band from which the resource allocation information is transmitted.
An LTE-A terminal can receive a plurality of component bands by receiving resource allocation information in this way. Here, an LTE terminal can receive only one component band at a time. In this way, bundling a plurality of component bands into an allocated band for a single communication is referred to as “carrier aggregation.” It is possible to improve throughput using this carrier aggregation.